ASME PTC-10 (2009) provides a test procedure to determine the thermodynamic performance of centrifugal compressors for gases conforming to ideal gas laws and for real gases. It requires using real gas calculation methods where the compressibility values depart from the specified limits. ASME PTC-10 employs Schultz X and Y compressibility factors to calculate the polytropic exponent for real gas compression. Specific values of X and Y for the test gas at the test condition may be different from the values provided in ASME PTC-10 generalized charts. Therefore, special care should be taken to properly calculate X and Y factors for a test gas at specified conditions.
In this paper, Schultz compressibility factors X and Y are derived as functions of reduced properties. These functions can be used with any equation of state to precisely calculate X and Y values for any gas composition at the specified operating conditions. By using the proposed method, Schultz X and Y compressibility factors for propane are graphically represented covering a reduced pressure range of 0.1 to 3 and a reduced temperature range of 1.05 to 2. Also, the rate of change of polytropic exponents for propane over a wide range of pressures and temperatures is graphically demonstrated.
